1. Technical Field
The present invention relates to a film-forming method and a film-forming device.
2. Related Art
Multi-layered substrates made of low temperature co-fired ceramics (LTCC) are widely used for a substrate of a high frequency module, a substrate of an IC package, and the like due to their excellent high-frequency property and high heat-resistance. As a method for manufacturing a film pattern of a wiring and the like included in the LTCC multi-layered substrates, an inkjet method has attracted attention in order to improve productivity and lower a cost. The inkjet method uses a droplet discharge head that discharges a liquid material including a wiring material as a droplet. In the method, the droplet discharge head is allowed to discharge a droplet while the droplet discharge head and a substrate are relatively moved in a main scanning direction. A plurality of droplets including the wiring material are sequentially united along the main scanning direction of the substrate so as to form a liquid film having a linear shape and continuing in the main scanning direction. In the inkjet method, the liquid film having a linear shape is dried so as to form a pattern.
JP-A-2005-152758 discloses a following inkjet method. In the method, a temperature gradient is applied on a surface of a linear liquid film and a high-temperature side end and a low-temperature side end are formed on both sides of the film across a main scanning direction. The liquid film having the temperature gradient forms surface tension distribution on its surface and generates Marangoni convection in its inside. A thermal capillary flow flowing out from the high-temperature side end of the liquid film descends toward a substrate before the flow reaches the low-temperature side end due to the temperature gradient applied to the liquid film. As a result, a wiring material that is not included in a flow path of Marangoni convection is separated out. Due to this wiring material separated out, the spread of the liquid film is pinned. On the other hand, the wiring material is continuously conveyed to the high-temperature side end by the convection, becoming hard to separate out the wiring material. Therefore, as the drying of the liquid film progresses, the high-temperature side end is constricted toward the low-temperature side end of the liquid film, separating out the wiring material only at the low-temperature side end of the liquid film. As a result, the liquid film forms a wiring pattern having a line width narrower than the film itself.
The inkjet method mentioned above has attracted attention also as a method for forming an orientation film that is used for a liquid crystal display, as shown in JP-A-2006-15271, for example. FIGS. 9A and 10A are plan views and FIGS. 9B and 10B are side views schematically showing a film-forming process of an orientation film. In the film-forming process of an orientation film, a droplet discharging treatment in which a droplet D is discharged on a substrate S so as to form a liquid film F0 and a drying treatment in which a solvent and the like included in the liquid film F0 are evaporated so as to dry the liquid film F0 are conducted.
As shown in FIGS. 9A and 9B, on a surface of the substrate S (hereinafter, referred to as a discharge surface Sa), in the droplet discharging treatment, a plurality of discharge regions R extending in a vertical direction are formed in a contiguous manner to be virtually separated in a horizontal direction. A droplet discharge head H moves above the discharge regions R in sequence from the discharge region R that is positioned leftmost along an arrow direction so as to discharge a plurality of droplets D including an orientation film material to the whole of each of the discharge regions R. Thus a partial liquid film F having a liner shape is formed on each of the plurality of discharge regions R. That is, the droplet discharge head H forms the liquid film F0 by multi-scanning. Alternatively, as shown in FIGS. 10A and 10B, a plurality of droplet discharge heads H arranged in a horizontal direction respectively discharge the droplets D on the whole of the discharge regions R so as to form the partial liquid film F on each of the plurality of discharge regions R. That is, the plurality of droplet discharge heads H form the liquid film F0 by single-scanning. Each of a plurality of partial liquid films F is united with adjacent partial liquid film F so as to form the liquid film F0 covering the whole of the substrate S.
In a case of film-forming by multi-scanning, landing timings of the droplets D are different from each other at a boundary between the partial liquid films F that are adjacent by a period of one scanning of the droplet discharge head. Further, even in a case of film-forming by single-scanning, landing timings of the droplets D are different from each other at a boundary between the partial liquid films F that are adjacent by a period between scans of the droplet discharge heads H that are formed with a certain distance.
At end parts (both end parts Fe in a horizontal direction, for example) of the partial liquid film F, a surface area per unit volume is large, so that evaporation probability of an evaporation component at the end parts increases and thus a drying speed becomes higher than that at a central part Fc. Therefore, flowage of the orientation film material occurs inside of the liquid material due to its increased viscosity, so that a concentration of the orientation film material becomes locally high at the both end parts Fe of the partial liquid film F. As a result, if the liquid film F0 is dried, difference in film thickness (contrasting density in FIGS. 9A and 10A) is disadvantageously formed at the both end parts Fe of the liquid film F0 after dried.